They aren't diecast but Ertl's Big Farm lineup includes (2) 1:16 excavators (John Deere and Hitachi) for around $50 each...
Saw a few of the Deere versions in Iowa at the National Toy Show this weekend.....to be honest the boom/stick seemed pretty flimsy but the cab and counterweight portions seemed like they'd make a nice base for a conversion.
-Alex